1. Field of the Invention
The invention relates generally to whipstocks for diverting the direction of drilling. More specifically, the invention relates to a method and apparatus for retrieving a whipstock from a wellbore.
2. Background Art
Traditionally, whipstocks have been used to drill a deviated borehole from an existing wellbore. A whipstock has a ramped surface which is set in a predetermined position to guide the drill bit or drill string in a deviated manner to drill into the side of the borehole. In operation, the whipstock is set on the bottom of the existing borehole, the set position of the whipstock is surveyed, and the whipstock is properly oriented for directing the drill string in the proper direction. After the whipstock is set, a drill string is lowered into the well into engagement with the whipstock causing the whipstock to orient the drill string to drill a deviated borehole through the wall of the existing borehole.
Other uses for whipstocks include instances where previously drilled and cased well bores, for one reason or another, have become nonproductive. For example, when a well bore becomes unusable, a new borehole may be drilled in the vicinity of the existing cased borehole or alternatively, a new borehole may be sidetracked from the serviceable portion of the existing cased wellbore. Sidetracking from a cased borehole may also be useful for developing multiple production zones. This procedure can be accomplished by milling through the side of the casing with a mill that is guided by a wedge or whipstock component.
After a milling or drilling procedure is completed, it is often desirable to retrieve the whipstock component from the borehole. Various tools are known in the art for retrieving whipstocks. Known methods of retrieval include using hooks, overshots, or die collars. Often, however, the whipstock incurs damage by the drilling or milling operations making retrieval by traditional means more difficult, if not impossible. Likewise, damage to a whipstock's profile can make deciding the preferred method of retrieval, and even the ability to retrieve the whipstock, less predictable. Finally, known retrieval tools and methods have a limited ability to retrieve a whipstock while utilizing a non rotary running processes, such as coil tubing.